The release marks the launch of Debug Fresno, a field study that aims to rid the central California county of Aedes aegypti mosquitoes. Also known as yellow fever mosquitoes, they first arrived in the area in 2013 and are known to spread the Zika virus, dengue, and chikungunya (although none of these viruses are currently spreading in Fresno).

Verily is working with Fresno's Consolidated Mosquito Abatement District to release 1 million male mosquitoes every week for 20 weeks, starting now. These mosquitoes have been rendered essentially sterile by infection with a bacteria called Wolbachia pipientis, which naturally colonizes mosquitoes and other insects in the wild. In time, if the local females continue to mate with the sterile males, the population should drop. The effort will ramp up to the full 1 million mosquito capacity over the next week, Kathleen Parkes, a Verily spokesperson, told The Verge in an email.

The mosquitoes being released are not genetically modified; Wolbachia means they don't have to be. The bacteria naturally infects many mosquito populations, though not in Fresno. That's why this just might work: Wolbachia infections render the male mosquitoes released in Fresno sterile to any female that doesn't have a Wolbachia infection herself. And the males can't transmit the disease to females, which means that with a native population of uninfected females, a whole bunch of sterile males just came to the sexual marketplace. Also, the males don't bite — so residents shouldn't be pestered by the extra mosquitoes. (Verily developed its own technology to sort the females, which do bite, from the males.)

The way Wolbachia works is like a lock-and-key system for mosquito reproduction, explains entomologist Stephen Dobson at the University of Kentucky, whose spinoff company MosquitoMate provided Verily with their breeding stock of Wolbachia-infected females. Since uninfected females don't have the corresponding key to unlock the male's Wolbachia-modified genes, the eggs from these mismatched couplings never develop.

This isn't Fresno's first rodeo with Wolbachia mosquitoes. MosquitoMate worked with CMAD to release about 800,000 Wolbachia-infected males in 2016. The Environmental Protection Agency determined that this pilot experiment wasn't particularly risky to anyone who wasn't a mosquito bent on reproduction. So, in September 2016, the EPA renewed the permit to continue, and expand, the experiment.